IPT systems are a well-known area of established technology (for example, wireless charging of electric toothbrushes) and developing technology (for example, wireless charging of handheld devices on a ‘charging mat’). Typically, a power transmitter generates a time-varying magnetic field from a transmitting coil or coils. This magnetic field induces an alternating current in a suitable receiving coil in a power receiver that can then be used to charge a battery, or power a device or other load.
Regarding IPT systems for wireless charging of handheld devices in particular it is important that the wireless power is transferred to the receiver device only and not to so-called foreign objects, which can be defined as any object that is positioned on the charging mat (e.g., interface surface), but is not part of a receiver device. Typical examples of such foreign objects are parasitic elements containing metals such as coins, keys, paperclips, etc. For example if a parasitic metal is close to the active IPT area it could heat up during power transfer due to eddy currents that result from the oscillating magnetic field. In order to prevent the temperature of such parasitic metal from rising to unacceptable levels, the power transmitter should be able to discriminate between power receivers and foreign objects and timely abort the power transfer.
A conventional manner of detecting heating of foreign objects on an interface surface uses a power loss method. In this method the received power PPR is used to indicate the total amount of power that is dissipated within the power receiver contained in the handheld device due to the magnetic field produced by the power transmitter. The received power equals the power that is available from the output of the power receiver plus any power that is lost in producing that output power. The power receiver communicates its PPR to the power transmitter so that the power transmitter can determine whether the power loss is within acceptable set limits, and if not, the power transmitter determines anomalous behaviour which may indicate presence of a foreign object and aborts power transmission. However, this power loss accounting method does not in itself provide actual detection of a foreign object, only the occurrence of non-expected behaviour.
International patent publication number WO2014/095722, by contrast, proposes a method of foreign object detection which uses excitation and detection coils within the transmitter, separate from the primary IPT transmitter coil(s). In that case either changes in the output voltage in the detection winding, or changes in the inductance of the detection winding are used to determine possible presence of an object. However this system requires a complex calibration to determine the base inductance. It is also insensitive to metal objects versus ferrous or magnetic objects, and therefore does not provide a means to discriminate between foreign objects and friendly objects, e.g., a receiver device. Any undesirable effects of operation of the primary IPT field on the detection is also not considered or characterised, such that the proposed method may be unreliable.